A DIAC is a two-terminal, bi-directional trigger device with no gate lead. Once a DIAC is conducting, what is the only way to switch it off?

Introduction / Context:
Unlike TRIACs and SCRs, DIACs have no gate terminal. They are used to provide symmetrical trigger pulses, commonly in phase-control circuits. Understanding how conduction starts and stops in a DIAC is key to designing dimmers and trigger networks.

Given Data / Assumptions:

• Two-terminal DIAC device.
• Bi-directional conduction once breakover voltage is reached.
• No external gate control is available.

Concept / Approach:
Conduction in a DIAC begins when the applied voltage magnitude exceeds its breakover voltage. Once conducting, it remains on until the current falls below the holding current. Because there is no gate, you cannot command turn-off with a gate signal; you must rely on the circuit current decreasing sufficiently—often naturally at AC zero crossings.

Step-by-Step Reasoning:
1) Turn-on: |V| exceeds breakover → DIAC switches to low-impedance state.2) On-state persists while device current ≥ holding current.3) Turn-off occurs only when current drops below holding current (low-current dropout).4) In AC circuits, this is normally achieved each half-cycle at the zero crossing.

Verification / Alternative check:
Review typical lamp dimmer circuits: DIAC triggers a TRIAC; both devices naturally commutate off when the AC load current passes through zero and falls below their holding currents.

Why Other Options Are Wrong:
Positive/negative gate voltages: DIAC has no gate terminal.

Increase voltage above breakover: that turns it on, not off.

Reverse with a gate pulse: again, no gate; polarity reversal alone does not guarantee turn-off unless current drops below holding current.

Common Pitfalls:
Confusing DIAC behavior with TRIAC/SCR gate control. Always design the surrounding network to guarantee current fall below holding current for reliable commutation.

Final Answer:
reduce current below its holding value (low-current dropout)